1. Field of the Invention
This invention relates to a method and apparatus for converting multiphase power of one frequency to multiphase power of another frequency. More particularly, it relates to a novel method and apparatus in which segments of an input current waveform are selectively switched to output lines to synthesize an output current waveform having the desired fundamental frequency.
2. Prior Art
In many applications, there is a need for an efficient power supply with a controllable output frequency. One such application is an airborne power generating system where the prime source of electrical power is a rotating generator that receives its mechanical power input from the engine of the aircraft. Since the engine speed varies, usually over a 2 to 1 range, it is not possible for the generator to produce constant frequency output if coupled directly to the engine. Heretofore the general practice has been to insert a hydraulic constantspeed coupling device between the engine and the generator, thereby enabling the generator to operate at a constant speed and hence produce electrical power at a constant frequency. Such a system has several disadvantages, not the least of which is relatively frequent and costly maintenance. These disadvantages can be overcome by an alternative approach in which the generator is directly coupled to the engine, allowing it to produce a variable frequency output power, as dictated by the engine speed. This variable frequency power is then converted into accurately regulated constant frequency output power by means of a static frequency converter.
Two basic types of static frequency converters have been proposed for aircraft power supply applications. One is a so-called DC link type converter, the other is a direct AC-to-AC frequency changer. In the DC link converter arrangement, the alternating voltage of the generator is converted first into a direct voltage by a rectifier circuit, then the direct voltage is converted back to alternating voltage, at the desired frequency, by a static power inverter. In the direct AC-to-AC frequency changer, the available variable frequency generator power is converted directly into constant frequency output power. Thus, the AC-to-AC frequency changer produces the desired output power in a single stage power conversion, whereas the DC link type converter necessitates double power conversion. For this reason, the frequency changer, in principle, has a higher operating efficiency and lower weight than its DC link type counterpart and, therefore, it appears to be the better solution for aircraft power generating systems or similar applications which require controllable frequency power supplies.
Various types of direct AC-to-AC frequency changers have been proposed and developed for aircraft power supply applications. These include the naturally commutated cycloconverter (NCC), the unrestricted frequency changer (UFC), and the unity displacement factor frequency changer (UDFFC). For a more detailed description of these frequency changers, refer to pages 384 to 395 of the book Static Power Frequency Changers, by L. Gyugyi and B. R. Pelly, John Wiley and Sons, Inc., 1976. Some additional novel frequency changer arrangements are described in U.S. Pat. No. 3,832,625 and in commonly owned U.S. Patent applications Ser. Nos. 292,975 and 293,045, both filed on Aug. 14, 1981 in the name of Laszlo Gyugyi, now U.S. Pat. Nos. 4,352,155 and 4,353,156 respectively.
The naturally commutated cycloconverter (NCC) employs controlled rectifier type semiconductors (SCRs) with no intrinsic turn-off capability. These devices are commutated (turned off) by the process of "natural commutation," by which the current is transferred without external forcing between the controlled rectifier type circuit elements. This is achieved by proper selection of the switching instants relative to the instantaneous polarities of the input voltages, when the output voltage waveform is synthesized. Natural commutation is desirable because controlled rectifier type devices are presently available with sufficiently high rating in small physical sizes. However, restrictions on output voltage waveform construction needed to satisfy the conditions for natural commutation result in a lagging input power factor (at any load power factor) and in the generation of harmonic components in the output that are difficult to filter. The lagging power factor increases the rating and size of the generator; and the harmonics necessitate a relatively large output filter. For these reasons, much of the weight and size benefits potentially obtainable with direct AC-to-AC power conversion cannot be realized.
All the other AC-to-AC frequency changers (UFC, UDFFC, etc.) require switching devices with intrinsic turn-off capability (e.g., transistors, gate turn-off thyristors, etc.) or an external force commutating circuit. These frequency changers overcome the shortcomings of the NCC and generally offer superior performance characteristics, for example, reduced output harmonic distortion, improved input power factor, etc. However, the types of frequency changers in which the switching devices are forcibly turned off, draw currents with step-like changes from the input source. Since the input source is a generator with an internal inductive impedance, the step-like current changes cause large voltage surges at the input terminals of the frequency changer, and thus across the (open) switching devices. These voltage surges could destroy the switching devices, cause other equipment damage and would also increase the output distortion. In order to limit the magnitudes of the voltage surges to some acceptable value, the prior art frequency changers employ a multiphase input filter arrangement, as proposed in U.S. Pat. No. 3,641,417, or energy absorbing "snubber" circuits.
The prior art frequency changers also require L-C filters to reduce harmonic distortion in the output voltage. The large inductors used in these filters also significantly increase the weight and size of the airborne power system.
In order to increase the power available with given switching devices, it is common to divide them into pulse groups so that more than one switch is supplying power to each output phase at one time. However, if this is done, it is necessary to couple the pulse groups to the common output phase with an interphase transformer since the instantaneous voltages that are being switched at the same time are not equal. If the switching devices are arranged in a bridge configuration, isolation transformers must be used in the output of the frequency changer for similar reasons. Again, in both cases, the transformers add to the weight and size of the power supply.
Basically, the main problem with the prior art frequency changers is that practical generators, in fact, most of the practical AC power sources, have an appreciable inductive source impedance at the relatively high switching frequencies at which the frequency changer is operated. This inductive source impedance resists any rapid change in the input current, that is, it tends to act similarly to a theoretical current source. The prior art frequency changer attempts to overcome this problem by using an input filter, or appropriate snubber circuit, to approximate a multiphase voltage source type termination at its input. However, the input filter can result in an appreciable weight penalty, and the snubber circuit can produce relatively high losses at the frequencies encountered in aircraft power supply applications.